Multiple reflector multiple frequency band antenna system



July 23, 1968 LA VERGNE wlLLIAMS ET AL 3,394,378

MULTIPLE REFLECTOR MULTIPLE FREQUENCY BAND ANTENNA SYSTEM Filed Nov. 16, 1964 W LE ms mm E WE GD RN EA mm LR ATTORNEY$ 3,394,378 MULTIPLE REFLECTOR MULTIPLE FREQUENCY BAND ANTENNA SYSTEM La Vergne E. Williams, Melbourne Beach, and Roland E.

Moseley, Iudialantic, Fla., assignors to Radiation Incorporated, Melbourne, Fla., a corporation of Florida Filed Nov. 16, 1964, Ser. No. 411,282 8 Claims. (Cl. 343-779) ABSTRACT OF THE DISCLOSURE An antenna system capable of transmitting or receiving RF energy of at least three different frequencies, in which a paraboloidal reflector has an axis of symmetry in common with that of an ellipsoidal subreflector, whose concave surface is opposite the concave surface of a paraboloidal reflector, and in common with the axis of a hyperboloidal subreflector positioned between the paraboloidal and ellipsoidal reflectors, the convex surface of the hyperboloidal reflector opposite the concave surface of the paraboloidal reflector. The latter operates in conjunction with the hyperboloidal subreflector in a Cassegrainian mode at the highest of the three frequencies, and in conjunction with the ellipsoidal subreflector in a Gregorian mode at the lowest of the three frequencies, and in the prime focus mode at the intermediate frequency.

The present invention relates generally to antennas, and, more particularly, to multiple-reflector antenna systems capable of simultaneous, high-performance operation in a plurality of independent frequency bands. Further, this invention allows operation of the various primary feeds in modes (prime focus, Cassegrainian, Gregorian, etc.) such that performance in all frequency bands can be maximized independently.

In many space communications antennas there exists requirements for simultaneous operation in a plurality of independent frequency bands; for example, 400 mc., 2000 mc., 4000 mc., and 8000 mc. Further based on such requirements as maximizing antenna gain to noise ratio, high power capacity, etc., it is often desirable to employ Cassegrainian or other multiple reflector aperture formation techniques. Considering the above frequency requirements example, location of the 400 me. primary feed in a Cassegrainian configuration would require a subreflector so large as to seriously degrade antenna performance at the higher frequencies.

Several dual reflector, dual frequency reflector-type antenna systems have previously been devised, but in general such systems involve the use of only one RF frequency, the other lying in a frequency range above the microwave region of the electromagnetic spectrum, typically in the infrared region. Most frequently such antennas are used as scanners in conjunction with radar systems to provide both active (transmission/reception) and passive (reception only) system operation. In such dual frequency systems, it is customary to provide some means of passing the microwave energy and of reflecting the infrared energy or vice versa to suitable detectors, and this is generally accomplished by a single reflector. By way of example, the reflector surface is suitably coated with an infrared reflecting material, such as by deposition of a plurality of alternate germanium and cryolyte layers or by a fixed coating pattern of infrared reflecting metal in bars or grid work perpendicular to the polarized electric field vector of the RF energy. Alternatively, the reflector surface may be made reflective to microwave energy and transmissive to infrared energy by, for example, the provision of a grid having relatively small openings therein in comparison to the wave length of the microwave energy,

ited States Patent ice such that the reflector has the appearance of a solid mirror to microwaves, but is at the same time transparent to infrared radiation.

Such systems as described above perform admirably for simultaneous operation and response to light waves and radio waves, but are of little value for antenna operation at dual RF frequencies. Further, attempts which have hitherto been made to provide suitable dual RF frequency multiple reflector-type antennas have met with little practical success. In one known form of dual RF frequency antenna system, there is employed a plurality of curved perforated plates in a stacked configuration which act simultaneously as a lens for frequencies radiated by a source at the convex side of the stacked plates and as a reflector for frequencies radiated by a source at the concave side. Such a system has disadvantages insofar as the number of plates required to provide the dual lens-reflection operation, and in the positioning of the radiators at either side of the stacked configuration.

In accordance with the present invention a multiple frequency band antenna system is provided which is not polarization sensitive, and which is simultaneously operable at two or more different frequency bands with substantially no interference therebetween. Such system operation is accomplished by utilization of several antenna feeds located on the concave side of a main reflector and coacting with two or more subreflectors in different operating modes for each frequency band of interest, and wherein at least one of the subreflector is transmissive to one or more of the frequency bands and reflective to the others, radiation from the feeds being collimated in a highly directive beam by the main reflector.

It is, accordingly, a principal object of the present invention to provide a multiple reflector, multiple frequency band antenna system.

It is another object of the present invention to .provide an improved reflector-type antenna system capable of simultaneous response to frequency bands separated by a factor of 4 or more with a minimum'of interference therebetween.

It is, further, an object of the present invention to provide an improved reflector-type antenna system having a main reflector and one or more subreflectors associated with two or more feeds, each operating in a different quasi-optical mode.

Other objects, features, and attendant advantages of the present invention will become apparent from a consideration of the following detailed description of specific embodiments thereof, especially when taken in conjunction with the accompanying drawings in which:

FIGURE 1 is a partially schematic and partially crosssectional diagrammatic view of one embodiment of the present invention.

FIGURE 2 illustrates an exemplary pattern of conductive areas in the subreflector of FIGURE 1.

FIGURE 3 is a partially schematic and partially crosssectional diagrammatic view of another embodiment of the present invention.

Referring now to the drawings, wherein like reference numerals refer to like components, the antenna system illustrated in FIGURE 1 has a first feed 20 acting as a radiator for a first frequency F and a second feed 21 acting as a radiator for a second frequency F The feeds may consist, for example, of horns suitably coupled to wave guides (not shown) for appropriate excitation. It is to be understood that while the following description will refer generally to transmission from the antenna system, reciprocity holds, and the system may also be used for reception of the RF energy, or a combination thereof. Both feeds are positioned on the conca-ve side of a main reflector 28, a subreflector 25 being interposed between the two feeds. Energy radiated by feed 20 is directed toward and illuminates the convex side of the subreflector while energy radiated by feed 21 is directed toward the concave side of the subreflector 25. The system will, of course, include suitable mounting and support apparatus and may be movable or fixed for tracking or nontracking operation. Suitable arrangements for providing driving and support of scanning or fixed antenna systems are well known, and, since such means form no part of the present invention, need not be further discussed.

As will hereinafter be explained, subreflector 25 is fabricated in such manner as to permit passage therethrough of energy at frequencies radiated by source 21, while it provides a reflective surface for energy at frequencies radiated by source 20. The main reflector 28 may, for example, have a paraboloidal shape, and has a conductive concave surface for reflecting RF energy therefrom in a known fashion. In addition, subreflector or secondary reflector 25 may, for example, have a hyperboloidal surface, and is positioned such that RF energy incident thereon from feed 20 is reflected therefrom as though such energy were emanating from a source located at the focus of the main reflector 28.

It is to be emphasized that the specific shapes or contours of the reflectors designated throughout this specification and illustrated in the accompanying drawings are purely exemplary, and that the principles which are herein set forth are applicable to any multiple reflector, multiple frequency band antenna system. Horn 21 may be positioned at the focus of the main reflector such that energy from both sources, reflected from the main reflector surface, is collimated in a substantially parallel beam of rays. Exemplary paths of energy radiated by the two sources are indicated in FIGURE l by the dotted lines and arrows. The illustrated antenna thus operates simultaneously in a dual-reflector mode for the highest frequency band, which is radiated by feed 20, and in the prime focus mode for the lower frequency band, which is radiated from the apex of feed 21 coincident with the focus of the main reflector.

Referring now to FIGURE 2, there is shown an enlarged view, over that illustrated in FIGURE 1, of the convex side of subreflector or secondary reflector 25. The subreflector may be of any suitable dielectric material, as for example fiberglass or styrofoam. The reflecting surface for the highest frequency is provided by suitable application of a plurality of conductive elements 35 to the convex surface of subreflector 25. The conductive elements are unconnected and are thus insulated from each other by gaps 36 in the exposed dielectric material. Any suitable method may be used for applying the conductive elements to the subreflector convex surface. Exemplary of such methods are the adhesive application of appropriate pieces of aluminum foil, or the spraying of silver paint in an appropriately masked pattern, or silk screen printing.

The conductive elements 35 preferably have a six-sided pattern and are placed in a honeycomb configuration. Such a positioning arrangement is preferred because it is symmetrical, approaches a circle, and permits the conductive elements to be nested together with a minimum of space therebetween. By virtue of such a conductive element pattern on the convex surface of the subreflector, the system is not polarization sensitive, and may be used with circular or orthogonal linear polarization. If the system is limited to linear polarization the conductive surfaces may alternatively consist of wire grids.

For simultaneous operation at the two frequencies, the conductive elements 35 should be at least one wavelength across at the highest frequency F The lower frequency F should differ from high frequency F by a factor sufficient to make the conductive elements appear less than A wavelength at this lower frequency. Thus, in the Cassegrainian mode, the subreflector is essentially a solid reflector which reflects microwave energy in the frequency band F toward the main reflector, while in the prime focus-fed mode of operation the subreflector is essentially transparent, and permits passage therethrough of microwave energy in the frequency band F for reflection from the main dish 28.

The location of the Cassegrainian mode feed 20 will depend almost entirely upon the shape and position of subreflector 25 and, within appropriate limits, may be placed almost anywhere on an axial line between the subreflector and a point behind the main reflector. In the latter case, the reflector 28 is provided with a small central hole behind which the feed 20 is located. Typically, however, feed 20 will be located approximately midway between main reflector 28 and subreflector 25.

Although all Cassegrainian-type systems have some feed blockage unless polarization rotation is employed, such blockage is not serious because subreflector 25 will typically be the diameter of the main reflector, and will hence only obstruct one hundredth, or less, of the aperture area. In this connection, it will be understood that the figures are not drawn to any scale, being exaggerated in certain areas for purposes of simplicity and clarity. Thus, in the practical situation feed blockage at the aperture of the main reflector will normally be only a few percent and will not appreciably reduce aperture efficiency.

Referring now to FIGURE 3, a second embodiment of the-present invention is illustrated, the system being similar in many respects to that of FIGURE 1. Feeds 20 and 21 are positioned and operate at frequencies as previously discussed. In addition, the positions and shapes of subreflector 25 and main reflector 28 are subject to the previous considerations. In this embodiment, however, the antenna system is arranged to operate at 3 different frequency bands. To this end, a third feed element 22 is positioned adjacent feed 20, and directs microwave energy toward subreflector 25. A second subreflector 26 is placed adjacent subreflector 25 such that the concave surfaces of the two subreflectors are opposite each other. Feed 22 and subreflector 26 are thus arranged to operate in conjunction with main reflector 28 in a Gregorian mode. The frequency band F of the microwave energy emanating from feed 22 should be several times lower than frequency band F Subreflector 25 will thus appear transparent to the F frequencies, permitting passage therethrough for illumination of second subreflector 26. Preferably, the latter is ellipsoidal in shape and has a conductive concave surface. Again, if a parallel beam of rays is desired to be reflected from main reflector 28, ellipsoidal subreflector 26 should be positioned such that the waves reflected therefrom appear to be emanating from a feed positioned at the focus of the paraboloidal reflector.

The location of feeds 20 and 22 will depend primarily upon the contours of the two subreflectors. They may be positioned at the same point, or may be separated as illustrated in FIGURE 3. Alternatively, since frequency bands F and F are widely different, a single dual frequency feed may conveniently be utilized in place of separate feeds 20 and 22. Feed 21 should be small with respect to the wave length of frequency F so that the F signal may pass by the feed with relatively little reflection or other disturbance.

The antenna system embodiment of FIGURE 3 operates in the following manner. In the Cassegrainian mode, microwave energy in the frequency band F is incident upon the convex surface of subreflector 25, and the surface presents a solid mirror to this frequency band as previously discussed. Thus, the F frequencies are reflected from the subreflector 25 toward main reflector 28. In the prime focus mode, microwave energy in the frequency band F passes directly through subreflector 25 since the latter is transmissive in this frequency band. In the Gregorian mode, subreflector 25 is also transmissive at frequencies F the latter frequencies being reflected from the ellipsoidal subreflector 26. Thus, all three frequency bands, F F F are collimated by paraboloidal reflector 28, the antenna system operating with substantially no interference between the widely different bands.

A wide variety of frequencies may be used for the various modes of operation of the antenna system, depending upon the size of the main reflector. By way of example, a 60 foot reflector may be employed for the frequencies F =2000 niegacycles, and F =200 megacycles. The only limitation placed upon the several frequencies which may be employed is that each reflector be at least 3 or more wave lengths across at the frequency for which it is to act as a reflector.

Again, it is to be emphasized that although the multiple frequency band antenna system has been described with reference to the transmitting of RF energy, such systems may be used for either transmitting or receiving or a combinijtion thereof.

While the present invention has been shown and described with reference to certain preferred embodiments thereof, it will be clear that various changes and modifications may be resorted to without departing from the true spirit and scope of the invention as defined by the appended claims.

We claim:

1. In a multiple frequency band antenna system,

a primary reflector having a concave surface for refleeting RF energy therefrom, a first subreflector and a second subreflector each having a convex surface and a concave surface,

said reflector and said subreflectors having a common axis of symmetry along which said first subreflector convex surface faces said primary reflector concave surface and said first subreflector concave surface faces said second subreflector concave surface,

said first subreflector having means for reflecting RF energy in the highest frequency band and for passing RF energy in the other frequency bands below said highest frequency band with which said antenna system is to be used.

2. The combination according to claim 1 wherein said primary reflector has a paraboloidal contour, said first subreflector has a hyperboloidal contour, and said second subreflector has an elliptical contour.

3. The combination according to claim 1 wherein said means comprises a plurality of conductive areas on one of said surfaces of said first subreflector,

each of said conductive areas having a lineal dimension of approximately one wavelength for the highest frequency band and of approximately one-quarter wavelength for the next lower frequency band with which said. antenna system is to be used.

4. The combination according to claim 3 wherein said conductive areas are hexagonal in shape and are disposed to form a. honeycomb pattern on the convex surface of said subreflector,

said areas insulated from each other by dielectric mate rial.

5. A highly directive, polarization insensitive antenna system capable of simultaneous translation of three diffrent RF frequencies, said system comprising at least three reflectors;

one of said reflectors for collimating RF energy at all of said frequencies;

said one reflector cooperating with another of said reflectors for operation at the highest of said three frequencies in a Cassegrainian mode; and cooperating with the third of said reflectors for operation at the lowest of said three frequencies in a Gregorian mode,

and operating at the intermediate of said three frequencies in the prime focus mode;

said another reflector transparent to frequencies other than said highest frequency and interposed between said one reflector and said third reflector along a common axis of symmetry.

6. An antenna system for translating RF energy of three different frequencies, comprising, in combination,

a main reflector having a concave conductive surface for reflecting RF energy,

first and second subreflectors each having a concave surface and a convex surface,

a plurality of unconnected conductive areas on one of said surfaces of said first subreflector,

each of said conductive areas having a lineal dimension of at least one wavelength at the highest of said frequencies and less than wavelength at the intermediate of said frequencies,

feed means for said highest frequency disposed with respect to said first subreflector and said main reflector for operation in a Cassegrainian mode,

feed means for said intermediate frequency disposed at the prime focus of said main reflector, feed means for the lowest of said frequencies disposed with respect to said second subreflector and said main reflector for operation in a Gregorian mode,

said first subreflector being reflective to RF energy at said highest frequency and transparent to RF energy at said intermediate and lowest frequencies, and

said second subreflector for reflecting RF energy at said lowest frequency.

7. The combination according to claim 6 wherein said feed means for said highest frequency and said feed means for said lowest frequency are combined in a dual frequency feed.

8. The combination according to claim 6 wherein said concave surface of said main reflector has a paraboloidal contour,

said first subreflector has a hyperboloidal contour with said convex surface thereof facing said concave surface of said main reflector and having a common axis therewith,

said second subreflector has an ellipsoidal contour with said concave surface thereof facing said concave surface of said first subreflector and having said common axis,

said highest frequency feed means and said lowest frequency feed means facing said convex surface of said first subreflector along said common axis in proximity to said main reflector, and

said intermediate frequency feed means facing said concave surface of said subreflector along said common axis, from the concave surface of said second subreflector.

References Cited UNITED STATES PATENTS 2,840,819 6/ 1958 McClella-m 343781 2,972,743 2/ 1961 Svensson et al 343781 3,148,370 9/ 1964 Bowman 343779 X 3,231,892 1/ 1966 Matson et al 343775 3,281,850 10/ 1966 Hannan 343779 X ELI LIEBERMAN, Primary Examiner.

W. H. PUNTER, Assistant Examiner. 

